BORON ISOTOPE COMPOSITION OF TOURMALINE, PRISMATINE AND GRANDIDIERITE IN GRANULITE-FACIES BORON-RICH ROCKS FROM THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA: NO EVIDENCE FOR A NON-MARINE EVAPORITE PRECURSOR

In contrast to most granulite-facies rocks, tourmaline (Tur) quartzite, prismatine (Prs)-bearing leucogneiss and grandidierite-bearing (Gdd) borosilicate gneiss in the Larsemann Hills, East Antarctica, have retained B in amounts up to 20 000 ppm. To constrain possible sources of B in the precursor, Tur, Prs and Gdd were analyzed in situ for B isotopes using secondary ion mass spectrometry with a Cameca ims 4f ion microprobe. δ¹¹B (= {[sample¹¹B/¹⁰B ¸ SRM 951¹¹B/¹⁰B] – 1} × 1000) averaged over several grains per sample ranges from –3.0 to –14.3‰ in Tur, from –9.6 to 18.1 ‰ in Prs and from –1.9 to –8.7 ‰ in Gdd (1s mostly 1-2‰ per sample). Tur in Tur quartzite is isotopically homogeneous from grain to grain in each sample and average Tur δ¹¹B varies little from sample to sample (6 samples: –5.9 to –8.8‰). Average leucogneiss Tur composition is lighter (3 samples, δ¹¹B = –9.6 to –14.3‰), whereas average borosilicate-gneiss Tur varies more (6 samples, δ¹¹B = –3.0 to –11.8‰). With two exceptions, average δ¹¹B increases in a given sample Prs < Tur < Gdd with Prs B 4.8±1.6‰ (9 pairs) lighter and Gdd B 2.8±1.9‰ (6 pairs) heavier than Tur B. This regularity suggests that the three borosilicates were in near isotopic equilibrium. In the anatectic pegmatites, δ¹¹B ranges from –4.8 to –12.1 ‰ in Tur (4 samples), –12.5 to –14.0‰ in Prs (2 samples) and –6.6 to –6.8‰ in Gdd (2 samples). These ranges lie within the corresponding ranges in the metamorphic rocks, i.e., the processes of melting and crystallization from melt together had little effect on B isotopic composition. Boron in Larsemann Hills Tur quartzite is at least 8‰ heavier than B in amphibolite-facies Tur quartzite from the Broken Hill district, Australia interpreted by Slack et al. (1993, Economic Geology, 88, 505-541) to be sourced from non-marine evaporite borate. Because ¹¹B tends to be lost during devolatilization with increasing metamorphic grade and the effect of melting is minor, the Larsemann Hills Tur B should be lighter than Broken Hill Tur B for the non-marine evaporite model to be supported. Thus, our B isotope data do not provide evidence for a role of non-marine evaporite in the precursor to the Larsemann Hills B-rich rocks. Possible precursors could be rocks altered by B-rich fluids associated with mud volcanoes such as those in the Kerch-Taman area north of the Black Sea.